1. INTRODUCTION

Two complementary approaches are necessary to understand galaxy
evolution: on the one hand, we need to develop theoretical models for
galaxy formation, chemical and dynamical evolution, and on the other
hand, we need to collect as many and as accurate as possible
observational data to constrain such models. In particular, we need to
know the masses, chemical abundances and kinematics of the various
galactic components, namely gas, stars and dark matter; we need to know
the star formation history (SFH), the initial mass function (IMF),
etc. In this review our current knowledge of the SFHs, as derived from
the colour-magnitude diagrams (CMDs) of their resolved stellar
populations, is summarized.

Resolved stellar populations are the best tracers of the SFH of a
galactic region, and their CMD the best tool to exploit the
tracers. This is due to the well known circumstance that the location of
any individual star in a CMD is uniquely related to its mass, age and
chemical composition. From the CMD we can thus disentangle directly
these evolution parameters. In the case of simple stellar populations,
i.e. coeval stars with the same chemical composition, isochrone fitting
is the most frequently used method to infer the system age. In the case
of galaxies, with rather complicated mixtures of different stellar
generations, the age determination is less straightforward, but their
CMDs remain the best means to derive the SFH.